Managing relocation of slices in storage systems

ABSTRACT

A method is used in managing data relocation in storage systems. Metadata of a slice of a storage tier in a data storage system is evaluated for migrating the slice from the storage tier to another storage tier. The data storage system includes a first storage tier and a second storage tier configured such that performance characteristics associated with the first storage tier is superior to the second storage tier. Based on the evaluation, relocation of the slice of the storage tier is effected. The metadata of the slice indicates whether the slice includes user data.

BACKGROUND

Technical Field

This application relates to managing data relocation in storage systems.

Description of Related Art

A traditional storage array (herein also referred to as a “data storagesystem”, “disk storage array”, “disk array”, or simply “array”) is acollection of hard disk drives operating together logically as a unifiedstorage device. Storage arrays are designed to store large quantities ofdata. Storage arrays typically include one or more storage arrayprocessors (SPs), for handling requests for allocation and input/output(I/O) requests. An SP is the controller for and primary interface to thestorage array.

A storage array may be thought of as a system for managing a largeamount of a resource, i.e., a large number of disk drives. Management ofthe resource may include allocation of a portion of the resource inresponse to allocation requests. In the storage array example, portionsof the storage array may be allocated to, i.e., exclusively used by,entities that request such allocation.

The administrator of a storage array may desire to operate the array ina manner that maximizes throughput and minimizes response time. Ingeneral, performance of a storage array may be constrained by bothphysical and temporal constraints. Examples of physical constraintsinclude bus occupancy and availability, excessive disk arm movement, anduneven distribution of load across disks. Examples of temporalconstraints include bus bandwidth, bus speed, spindle rotational speed,serial versus parallel access to multiple read/write heads, and the sizeof data transfer buffers.

Large storage arrays today manage many disks that are not identical.Storage arrays use different types of disks and group the like kinds ofdisks into tiers based on the performance characteristics of the disks.A group of fast but small disks may be a fast tier (also referred to as“higher tier” or “high tier”). A group of slow but large disks may be aslow tier (also referred to as “lower tier” or “low tier”). It may bepossible to have different tiers with different properties orconstructed from a mix of different types of physical disks to achieve aperformance or price goal. Storing often referenced, or hot, data on thefast tier and less often referenced, or cold, data on the slow tier maycreate a more favorable customer cost profile than storing all data on asingle kind of disk.

A storage tier may be made up of different types of disks, i.e., diskswith different redundant array of inexpensive disks (RAID) levels,performance and cost characteristics. In the industry there have becomedefined several levels of RAID systems. RAID (Redundant Array ofIndependent or Inexpensive Disks) parity schemes may be utilized toprovide error detection during the transfer and retrieval of data acrossa storage system.

Data storage systems, such as disk drives, disk storage arrays, networkstorage devices, storage area networks, and the like, are called upon tostore and manage a significant amount of data (e.g., gigabytes,terabytes, petabytes, etc.) that is written and read by many users. Forexample, a traditional storage array may include a collection of harddisk drives operating together logically as a unified storage device.Storage arrays are typically used to provide storage space for aplurality of computer file systems, databases, applications, and thelike. For this and other reasons, it is common for physical storagearrays to be logically partitioned into chunks of storage space, calledlogical units, or LUs. This allows a unified storage array to appear asa collection of separate file systems, network drives, and/or volumes.

SUMMARY OF THE INVENTION

A method is used in managing data relocation in storage systems.Metadata of a slice of a storage tier in a data storage system isevaluated for migrating the slice from the storage tier to anotherstorage tier. The data storage system includes a first storage tier anda second storage tier configured such that performance characteristicsassociated with the first storage tier is superior to the second storagetier. Based on the evaluation, relocation of the slice of the storagetier is effected. The metadata of the slice indicates whether the sliceincludes user data.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the present technique will become moreapparent from the following detailed description of exemplaryembodiments thereof taken in conjunction with the accompanying drawingsin which:

FIGS. 1-2 are examples of an embodiment of a computer system that mayutilize the techniques described herein;

FIG. 3 is an example illustrating storage device layout;

FIGS. 4A and 4B are examples illustrating storage device layout;

FIGS. 5-10 are block diagrams illustrating in more detail componentsthat may be used in connection with techniques herein; and

FIG. 11 is a flow diagram illustrating processes that may be used inconnection with techniques herein.

DETAILED DESCRIPTION OF EMBODIMENT(S)

Described below is a technique for use in managing data relocation instorage systems, which technique may be used to provide, among otherthings, evaluating metadata of a slice of a storage tier in a datastorage system for migrating the slice from the storage tier to anotherstorage tier, where the data storage system includes a first storagetier and a second storage tier configured such that performancecharacteristics associated with the first storage tier is superior tothe second storage tier, and based on the evaluation, effectingrelocation of the slice of the storage tier, where the metadata of theslice indicates whether the slice includes user data.

Generally, a storage pool is a collection of storage that is provisionedfor a logical unit. A storage pool may be a collection of disks, whichmay include disks of different types. Storage pools may further besubdivided into slices; for example, a 1 gigabyte (GB) slice may be theallocation element for a logical unit. Further, a slice may be 256megabytes (MB) in size. A pool may include a set of storage tiers. Astorage tier may include storage devices of similar or same performancecapabilities and cost. However, a pool may have storage devices ofdifferent performance capabilities and costs. Both pool and storage tiercontain slices. A slice may be considered the smallest element that canbe tracked and moved. It may be advantageous to store the hot or mostaccessed data on the devices within the storage pool with the bestperformance characteristics while storing the cold or least accesseddata on the devices that have slower performance characteristics. Thiscan lead to a lower cost system having both faster and slower devicesthat can emulate the performance of a more expensive system having onlyfaster storage devices.

A storage tier or a storage pool may be a collection of storagecontainers. A storage container may be a unit of storage including a setof storage extents. A storage extent is a logical contiguous area ofstorage reserved for a user requesting the storage space. For example, astorage tier may include three storage containers, each storagecontainer including a set of disks and the set of disk in each storagecontainer having different RAID levels.

A disk may be a physical disk within the storage system. A LUN may be alogical unit number which is an identifier for a Logical Unit. Eachslice of data may have a mapping to the location of the physical drivewhere it starts and ends.

Generally, slices are allocated to LUNs in a storage pool as “best-fit”at initial allocation time. In at least some cases, since the I/O loadpattern of a slice is not known at initial allocation time, theperformance capability of storage allocated may be too high or too lowfor effective data access on a slice. Furthermore, a data access patterntends to change over time. Older data is accessed less frequently andtherefore in at least many cases does not require storage with higherperformance capability. Temperature of each storage slice is anindication of hotness of a slice, in other words, frequency and recencyof slice I/Os. Better overall system performance can be achieved byplacing hot slices to higher tier and cold slices to lower tier.

Slice relocation (herein also referred to as a “data relocation” or“data migration”) is a process of determining optimal or near optimaldata placement among storage objects (e.g., storage tier, RAID group)based on I/O load of the storage objects. Slice relocation helps providea way to determine respective preferable or best storage locations ofslices within a LUN in a storage pool, and to construct a slicerelocation candidate list to move slices from their current locations tothe respective preferable or best locations. Data migration, i.e., themoving of data from one storage element to another, may be performed atthe LUN level or at the slice level. Data migration at the slice levelmay be performed by copying the data of a slice and then updating anaddress map of the slice with the new location of the slice. A slice maystore data or metadata of the data. I/O operations performed for copyingdata of a slice in order to relocate the slice are referred to asrelocation I/Os.

Further, a tiered storage pool may include storage with differentperformance characteristics such that a logical unit created fromstorage space provisioned from the storage pool may include slices fromdifferent storage tiers with different performance characteristics.Based on configuration of a storage pool and the type of a logical unitof the storage pool, slices may be provisioned for the logical uniteither dynamically at the time the logical unit requires slices forallocating storage space or at the time the logical unit is created.Allocating a slice to a logical unit is referred to as provisioning theslice to the logical unit. Thus, a provisioned slice allocated to alogical unit has an owner which may be a file system represented by thelogical unit. When a provisioned slice is written to by a host systemand includes user data, the provisioned slice is referred to as anallocated provisioned slice. When a provisioned slice has not beenwritten to by a host system and does not include any user data, theprovisioned slice is referred to as an unused provisioned slice. A sliceresiding in a storage pool which is available for provisioning to alogical unit is referred to as an un-provisioned slice. Further, a slicethat is provisioned to a logical unit but not yet written to by a hostsystem with user data is referred to as an empty slice.

Further, a pool of storage devices may be organized into multiple RAIDgroups, and each RAID group may further divided be into a number of LUsfrom which slices are allocated to one or more mapped LUs for use byusers of a storage array. As used herein, a mapped LU refers to alogical portion of storage space that represent contiguous and/ornon-contiguous physical storage space, where mapping allows for physicalstorage space to be dynamically linked together at a time of use into alogically contiguous address space. Exemplary examples of mapped LUs mayinclude thin logical units (TLUs) and direct logical units (DLUs). Athin logical unit (“TLU”) is a sparsely populated logical unit (LU)provisioned at creation but which is not allocated any storage until thestorage is actually needed. A “direct logical unit” or “DLU” (alsoreferred to as “direct mapped LUN”) is a fully provisioned mapped LUwith coarse mapping. Even though a DLU is seen as fully provisioned by auser, internally storage space is allocated on as needed basis. TLUs mayhave a logical size that is larger than the actual storage size consumedby the TLUs. The actual consumed size is determined by the number ofslices actually allocated to a TLU. Thus, an amount of storage spacepresented to a host of a data storage system using a thin logical volumemay be different than the amount of storage space actually allocated tothe thin logical volume. The slices that are allocated to a mapped LUNmay be physically located anywhere in a storage array.

Thus, when a DLU is completely provisioned at the time the DLU iscreated; either all or most of the slices of the DLU may be empty at thetime the DLU is created. Further, when a slice is provisioned to TLUdynamically upon a request to write data to the TLU, the TLU may includefew empty slices.

An automated storage tiering process (also referred to herein simply as“slice relocation process”) relocates slices among storage tiers inorder to improve I/O performance, decrease system runtime cost andreduce disk drive wear. However, the process of slice relocationconsumes system resources such as CPU, memory, cache space, andbandwidth of a backend storage device. Thus, it may be desirable torelocate slices of a storage system efficiently with least or reducedimpact on I/O performance of the storage system.

In a conventional system, an automated storage tiering process relocatesslices without evaluating whether a slice that is being relocated is anempty slice or includes user data. As a result, in such a conventionalsystem, when an empty slice is relocated from a hot storage tier to acold storage tier in order to accommodate new active slices (alsoreferred to herein as “hot” slices) in the hot storage tier, entirecontents of the empty slice is copied to a destination slice in the coldstorage tier even though data represented by the contents of the emptyslice is invalid. Further, generally, an empty slice may either berelocated by the automated storage tiering process dynamically to createadditional storage space in a storage tier or when a user changes atiering preference of a logical unit including the empty slice. Thus, insuch a conventional system, copying contents of an empty slice whenrelocating the empty slice consumes processing cycles of a CPU of astorage system and bandwidth of backend storage disks, increases theoverall amount of time required to relocate slices, and increases wearand tear of disk drives of a storage system. Further, in such aconventional system, data of a slice is relocated even when the slicedoes not include any user data. Further, in such a conventional system,empty slices that are provisioned to logical units but not yet writtenmay be considered as cold slices because the empty slices are not beingused by a host to write user data thereby becoming susceptible torelocation to cold storage tiers. Thus, in such a conventional system,empty provisioned slices that are cold based on temperature may berelocated to a cold storage tier during load balancing process. Further,in such a conventional system, empty provisioned slices that are coldbased on temperature may be relocated when a user changes tieringpreference of a logical unit including the empty provisioned slices.Consequently, in such a conventional system, copying of entire contentsof empty provisioned slices that are cold results in copying of a largeamount of either invalid data or data including zeros (e.g. gigabytes ofdata) thereby resulting in consumption of a large amount of storagesystem resources.

By contrast, in at least some implementations in accordance with thetechnique as described herein, the current technique evaluates whether aslice is an empty slice and avoids copying contents of the empty sliceduring a slice relocation process. Thus, in at least one embodiment ofthe current technique, overall storage system performance is increasedby identifying empty slices during relocation of slices and avoidingcopying of data when relocating an empty slice. Thus, in case of a DLUwhere entire storage space for the DLU is provisioned using slices froma storage pool at the time DLU is created, either all or most of theslices are empty slices. Thus, in such a case, using the currenttechnique, avoiding copying data of empty slices of a DLU significantlyimproves performance of a slice relocation process and reduces overallimpact of the slice relocation process on a storage system.

In at least one embodiment of the current technique, metadata such as abit (also referred to herein as “empty slice bit”) is added to a slicemap entry metadata associated with a slice indicating whether valid userdata has been written to the slice. When a slice is provisioned to alogical unit, the empty slice bit for the slice is set to zeroindicating that the slice is an empty slice. Further, when data is firstwritten to a slice, the empty slice bit for the slice is set to oneindicating that the slice includes valid user data. Further, in at leastone embodiment of the current technique, the empty slice bit for a slicemay be maintained in a memory of a storage system in order toefficiently evaluate metadata information for the slice without havingto read the metadata information from a disk. During relocation of aslice, a determination is made as to whether the empty slice bit for theslice is set indicating that the slice is an empty slice. Upondetermining that a slice is an empty slice, copying of contents of theslice is skipped and metadata of a destination slice is updated toindicate that the slice has been relocated without having to copy thecontents of the slice.

In at least some implementations in accordance with the currenttechnique as described herein, the use of the managing data relocationin storage systems technique can provide one or more of the followingadvantages: lowering storage costs by improving efficiency of the datastorage system, and improving I/O performance of a storage system byefficiently relocating slices and avoiding copying of invalid and/orzero data of empty slices.

Referring now to FIG. 1, shown is an example of an embodiment of acomputer system that may be used in connection with performing thetechnique or techniques described herein. The computer system 10includes one or more data storage systems 12 connected to host systems14 a-14 n through communication medium 18. The system 10 also includes amanagement system 16 connected to one or more data storage systems 12through communication medium 20. In this embodiment of the computersystem 10, the management system 16, and the N servers or hosts 14 a-14n may access the data storage systems 12, for example, in performinginput/output (I/O) operations, data requests, and other operations. Thecommunication medium 18 may be any one or more of a variety of networksor other type of communication connections as known to those skilled inthe art. Each of the communication mediums 18 and 20 may be a networkconnection, bus, and/or other type of data link, such as hardwire orother connections known in the art. For example, the communicationmedium 18 may be the Internet, an intranet, network or other wireless orother hardwired connection(s) by which the host systems 14 a-14 n mayaccess and communicate with the data storage systems 12, and may alsocommunicate with other components (not shown) that may be included inthe computer system 10. In at least one embodiment, the communicationmedium 20 may be a LAN connection and the communication medium 18 may bean iSCSI or fibre channel connection.

Each of the host systems 14 a-14 n and the data storage systems 12included in the computer system 10 may be connected to the communicationmedium 18 by any one of a variety of connections as may be provided andsupported in accordance with the type of communication medium 18.Similarly, the management system 16 may be connected to thecommunication medium 20 by any one of variety of connections inaccordance with the type of communication medium 20. The processorsincluded in the host computer systems 14 a-14 n and management system 16may be any one of a variety of proprietary or commercially availablesingle or multi-processor system, such as an Intel-based processor, orother type of commercially available processor able to support trafficin accordance with each particular embodiment and application.

It should be noted that the particular examples of the hardware andsoftware that may be included in the data storage systems 12 aredescribed herein in more detail, and may vary with each particularembodiment. Each of the host computers 14 a-14 n, the management system16 and data storage systems may all be located at the same physicalsite, or, alternatively, may also be located in different physicallocations. In connection with communication mediums 18 and 20, a varietyof different communication protocols may be used such as SCSI, FibreChannel, iSCSI, FCoE and the like. Some or all of the connections bywhich the hosts, management system, and data storage system may beconnected to their respective communication medium may pass throughother communication devices, such as a Connectrix or other switchingequipment that may exist such as a phone line, a repeater, a multiplexeror even a satellite. In at least one embodiment, the hosts maycommunicate with the data storage systems over an iSCSI or fibre channelconnection and the management system may communicate with the datastorage systems over a separate network connection using TCP/IP. Itshould be noted that although FIG. 1 illustrates communications betweenthe hosts and data storage systems being over a first connection, andcommunications between the management system and the data storagesystems being over a second different connection, an embodiment may alsouse the same connection. The particular type and number of connectionsmay vary in accordance with particulars of each embodiment.

Each of the host computer systems may perform different types of dataoperations in accordance with different types of tasks. In theembodiment of FIG. 1, any one of the host computers 14 a-14 n may issuea data request to the data storage systems 12 to perform a dataoperation. For example, an application executing on one of the hostcomputers 14 a-14 n may perform a read or write operation resulting inone or more data requests to the data storage systems 12.

The management system 16 may be used in connection with management ofthe data storage systems 12. The management system 16 may includehardware and/or software components. The management system 16 mayinclude one or more computer processors connected to one or more I/Odevices such as, for example, a display or other output device, and aninput device such as, for example, a keyboard, mouse, and the like. Adata storage system manager may, for example, view information about acurrent storage volume configuration on a display device of themanagement system 16. The manager may also configure a data storagesystem, for example, by using management software to define a logicalgrouping of logically defined devices, referred to elsewhere herein as astorage group (SG), and restrict access to the logical group.

It should be noted that although element 12 is illustrated as a singledata storage system, such as a single data storage array, element 12 mayalso represent, for example, multiple data storage arrays alone, or incombination with, other data storage devices, systems, appliances,and/or components having suitable connectivity, such as in a SAN, in anembodiment using the techniques herein. It should also be noted that anembodiment may include data storage arrays or other components from oneor more vendors. In subsequent examples illustrated the techniquesherein, reference may be made to a single data storage array by avendor, such as by EMC Corporation of Hopkinton, Mass. However, as willbe appreciated by those skilled in the art, the techniques herein areapplicable for use with other data storage arrays by other vendors andwith other components than as described herein for purposes of example.

An embodiment of the data storage systems 12 may include one or moredata storage systems. Each of the data storage systems may include oneor more data storage devices, such as disks. One or more data storagesystems may be manufactured by one or more different vendors. Each ofthe data storage systems included in 12 may be inter-connected (notshown). Additionally, the data storage systems may also be connected tothe host systems through any one or more communication connections thatmay vary with each particular embodiment and device in accordance withthe different protocols used in a particular embodiment. The type ofcommunication connection used may vary with certain system parametersand requirements, such as those related to bandwidth and throughputrequired in accordance with a rate of I/O requests as may be issued bythe host computer systems, for example, to the data storage systems 12.

It should be noted that each of the data storage systems may operatestand-alone, or may also included as part of a storage area network(SAN) that includes, for example, other components such as other datastorage systems.

Each of the data storage systems of element 12 may include a pluralityof disk devices or volumes. The particular data storage systems andexamples as described herein for purposes of illustration should not beconstrued as a limitation. Other types of commercially available datastorage systems, as well as processors and hardware controlling accessto these particular devices, may also be included in an embodiment.

Servers or host systems, such as 14 a-14 n, provide data and accesscontrol information through channels to the storage systems, and thestorage systems may also provide data to the host systems also throughthe channels. The host systems do not address the disk drives of thestorage systems directly, but rather access to data may be provided toone or more host systems from what the host systems view as a pluralityof logical devices or logical volumes. The logical volumes may or maynot correspond to the actual disk drives. For example, one or morelogical volumes may reside on a single physical disk drive. Data in asingle storage system may be accessed by multiple hosts allowing thehosts to share the data residing therein. A LUN (logical unit number)may be used to refer to one of the foregoing logically defined devicesor volumes. An address map kept by the storage array may associate hostsystem logical address with physical device address.

In such an embodiment in which element 12 of FIG. 1 is implemented usingone or more data storage systems, each of the data storage systems mayinclude code thereon for performing the techniques as described herein.In following paragraphs, reference may be made to a particularembodiment such as, for example, an embodiment in which element 12 ofFIG. 1 includes a single data storage system, multiple data storagesystems, a data storage system having multiple storage processors, andthe like. However, it will be appreciated by those skilled in the artthat this is for purposes of illustration and should not be construed asa limitation of the techniques herein. As will be appreciated by thoseskilled in the art, the data storage system 12 may also include othercomponents than as described for purposes of illustrating the techniquesherein.

The data storage system 12 may include any one or more different typesof disk devices such as, for example, an ATA disk drive, FC disk drive,and the like. Thus, the storage system may be made up of physicaldevices with different physical and performance characteristics (e.g.,types of physical devices, disk speed such as in RPMs), RAID levels andconfigurations, allocation of cache, processors used to service an I/Orequest, and the like.

Given the different performance characteristics, one or more tiers ofstorage devices may be defined. The physical devices may be partitionedinto tiers based on the performance characteristics of the devices;grouping similar performing devices together. An embodiment using thetechniques herein may define a hierarchy of multiple tiers. Conversely,the particular performance characteristics may be applied to a storagepool with or without the definition of tiers. The set of resourcesassociated with or designated for use by a tier or grouping within apool may be characterized as a dynamic binding in that the particularset of data storage system resources utilized by consumers in a tier mayvary from time to time. A current configuration for the data storagesystem, static aspects of the current data storage system resources(e.g., types of devices, device storage capacity and physical devicecharacteristics related to speed and time to access data stored on thedevice), and current workload and other dynamic aspects (e.g., actualobserved performance and utilization metrics) of the data storage systemmay vary at different points in time.

An Auto-Tiering policy engine (PE) of the data storage system 12examines a storage pool's storage configuration and temperatures of allslices in that storage pool, and generates a slice relocation list. Theslice relocation list identifies slices to be relocated with respectivedestination information. In general, slices in a storage pool arematched to the most appropriate respective tiers based on theirrespective temperatures (e.g., hot, cold) and tier preferences (e.g.,High, Low, Optimal). If a slice's current tier differs from its matchingtier, the slice is listed in the relocation candidate list. The PE isalso referred to herein as the slice relocation process.

In certain cases, an enterprise can utilize different types of storagesystems to form a complete data storage environment. In one arrangement,the enterprise can utilize both a block based storage system and a filebased storage hardware, such as a VNX™ or VNXe™ system (produced by EMCCorporation, Hopkinton, Mass.). In such an arrangement, typically thefile based storage hardware operates as a front-end to the block basedstorage system such that the file based storage hardware and the blockbased storage system form a unified storage system.

Referring now to FIG. 2, shown is an example of an embodiment of acomputer system such as a unified data storage system that may be usedin connection with performing the technique or techniques describedherein. As shown, the unified data storage system 10 includes a blockbased storage system 12 and file based storage hardware 34. While theblock based storage system 12 may be configured in a variety of ways, inat least one embodiment, the block based storage system 12 is configuredas a storage area network (SAN), such as a VNX™ or VNXe™ system, asproduced by EMC Corporation of Hopkinton, Mass. While the file basedstorage hardware 34 may be configured in a variety of ways, in at leastone embodiment, the file based storage hardware 34 is configured as anetwork attached storage (NAS) system, such as a file server systemproduced by EMC Corporation of Hopkinton, Mass., configured as a headerto the block based storage system 12.

The computer system 10 includes one or more block based data storagesystems 12 connected to host systems 14 a-14 n through communicationmedium 18. The system 10 also includes a management system 16 connectedto one or more block based data storage systems 12 through communicationmedium 20. In this embodiment of the computer system 10, the managementsystem 16, and the N servers or hosts 14 a-14 n may access the blockbased data storage systems 12, for example, in performing input/output(I/O) operations, data requests, and other operations. The communicationmedium 18 may be any one or more of a variety of networks or other typeof communication connections as known to those skilled in the art. Eachof the communication mediums 18 and 20 may be a network connection, bus,and/or other type of data link, such as a hardwire or other connectionsknown in the art. For example, the communication medium 18 may be theInternet, an intranet, network or other wireless or other hardwiredconnection(s) by which the host systems 14 a-14 n may access andcommunicate with the block based data storage systems 12, and may alsocommunicate with other components (not shown) that may be included inthe computer system 10. In one embodiment, the communication medium 20may be a LAN connection and the communication medium 18 may be an iSCSIor fibre channel connection.

Each of the host systems 14 a-14 n and the block based data storagesystems 12 included in the computer system 10 may be connected to thecommunication medium 18 by any one of a variety of connections as may beprovided and supported in accordance with the type of communicationmedium 18. Similarly, the management system 16 may be connected to thecommunication medium 20 by any one of variety of connections inaccordance with the type of communication medium 20. The processorsincluded in the host computer systems 14 a-14 n and management system 16may be any one of a variety of proprietary or commercially availablesingle or multiprocessor system, such as an Intel-based processor, orother type of commercially available processor able to support trafficin accordance with each particular embodiment and application.

In at least one embodiment of the current technique, block based datastorage system 12 includes multiple storage devices 40, which aretypically hard disk drives, but which may be tape drives, flash memory,flash drives, other solid state drives, or some combination of theabove. In at least one embodiment, the storage devices may be organizedinto multiple shelves 44, each shelf containing multiple devices. In theembodiment illustrated in FIG. 1, block based data storage system 12includes two shelves, Shelf1 44A and Shelf2 44B; Shelf1 44A containseight storage devices, D1-D8, and Shelf2 also contains eight storagedevices, D9-D16.

Block based data storage system 12 may include one or more storageprocessors 46, for handling input/output (I/O) requests and allocations.Each storage processor 46 may communicate with storage devices 40through one or more data buses 48. In at least one embodiment, blockbased data storage system 12 contains two storage processors, SP1 46A,and SP2 46B, and each storage processor 46 has a dedicated data bus 48for each shelf 44. For example, SP1 46A is connected to each storagedevice 40 on Shelf1 44A via a first data bus 48A and to each storagedevice 40 on Shelf2 44B via a second data bus 48B. SP2 46B is connectedto each storage device 40 on Shelf1 44A via a third data bus 48C and toeach storage device 40 on Shelf2 44B via a fourth data bus 48D. In thismanner, each device 40 is configured to be connected to two separatedata buses 48, one to each storage processor 46. For example, storagedevices D1-D8 may be connected to data buses 48A and 48C, while storagedevices D9-D16 may be connected to data buses 48B and 48D. Thus, eachdevice 40 is connected via some data bus to both SP1 46A and SP2 46B.The configuration of block based data storage system 12, as illustratedin FIG. 1, is for illustrative purposes only, and is not considered alimitation of the current technique described herein.

In addition to the physical configuration, storage devices 40 may alsobe logically configured. For example, multiple storage devices 40 may beorganized into redundant array of inexpensive disks (RAID) groups.Although RAID groups are composed of multiple storage devices, a RAIDgroup may be conceptually treated as if it were a single storage device.As used herein, the term “storage entity” may refer to either a singlestorage device or a RAID group operating as a single storage device.

Storage entities may be further sub-divided into logical units. A singleRAID group or individual storage device may contain one or more logicalunits. Each logical unit may be further subdivided into portions of alogical unit, referred to as “slices”. In the embodiment illustrated inFIG. 1, storage devices D1-D5, is sub-divided into 3 logical units, LU142A, LU2 42B, and LU3 42C. The LUs 42 may be configured to store a datafile as a set of blocks striped across the LUs 42.

The unified data storage system 10 includes a file based storagehardware 34 that includes at least one data processor 26. The dataprocessor 26, for example, may be a commodity computer. The dataprocessor 26 sends storage access requests through physical data link 36between the data processor 26 and the block based storage system 12. Thedata link 36 may be any one or more of a variety of networks or othertype of communication connections as known to those skilled in the art.The processor included in the data processor 26 may be any one of avariety of proprietary or commercially available single ormultiprocessor system, such as an Intel-based processor, or other typeof commercially available processor able to support traffic inaccordance with each particular embodiment and application. Further,file based storage hardware 34 may further include control station 30and additional data processors (such as data processor 27) sharingstorage device 40. A dual-redundant data link 32 interconnects the dataprocessors 26, 27 to the control station 30. The control station 30monitors a heartbeat signal from each of the data processors 26, 27 inorder to detect a data processor failure. If a failed data processorcannot be successfully re-booted, the control station 30 will “fenceoff” the failed data processor and re-assign or fail-over the dataprocessing responsibilities of the failed data processor to another dataprocessor of the file based storage hardware 34. The control station 30also provides certain server configuration information to the dataprocessors 26, 27. For example, the control station maintains a bootconfiguration file accessed by each data processor 26, 27 when the dataprocessor is reset.

The data processor 26 is configured as one or more computerized devices,such as file servers, that provide end user devices (not shown) withnetworked access (e.g., NFS and CIFS facilities) to storage of the blockbased storage system 12. In at least one embodiment, the control station30 is a computerized device having a controller, such as a memory andone or more processors. The control station 30 is configured to providehardware and file system management, configuration, and maintenancecapabilities to the data storage system 10. The control station 30includes boot strap operating instructions, either as stored on a localstorage device or as part of the controller that, when executed by thecontroller following connection of the data processor 26 to the blockbased storage system 12, causes the control station 30 to detect theautomated nature of a file based storage hardware installation processand access the data processor 26 over a private internal managementnetwork and execute the file based hardware installation process.

Referring to FIG. 3, shown is an example representing how data storagesystem best practices may be used to form storage pools. The example 50illustrates how storage pools may be constructed from groups of physicaldevices. For example, RAID Group1 64 a may be formed from physicaldevices 60 a. The data storage system best practices of a policy mayspecify the particular disks and configuration for the type of storagepool being formed. For example, for physical devices 60 a on a firstdata storage system type when forming a storage pool, RAID-5 may be usedin a 4+1 configuration (e.g., 4 data drives and 1 parity drive). TheRAID Group 1 64 a may provide a number of data storage LUNs 62 a. Anembodiment may also utilize one or more additional logical device layerson top of the LUNs 62 a to form one or more logical device volumes 61 a.The particular additional logical device layers used, if any, may varywith the data storage system. It should be noted that there may not be a1-1 correspondence between the LUNs of 62 a and the volumes of 61 a. Ina similar manner, device volumes 61 b may be formed or configured fromphysical devices 60 b. The storage pool 1 of the example 50 illustratestwo RAID groups being used to define a single storage pool although,more generally, one or more RAID groups may be used for form a storagepool in an embodiment using RAID techniques.

The data storage system 12 may also include one or more mapped devices70-74. A mapped device (e.g., “thin logical unit”, “direct logicalunit”) presents a logical storage space to one or more applicationsrunning on a host where different portions of the logical storage spacemay or may not have corresponding physical storage space associatedtherewith. However, the mapped device is not mapped directly to physicalstorage space. Instead, portions of the mapped storage device for whichphysical storage space exists are mapped to data devices such as devicevolumes 61 a-61 b, which are logical devices that map logical storagespace of the data device to physical storage space on the physicaldevices 60 a-60 b. Thus, an access of the logical storage space of themapped device results in either a null pointer (or equivalent)indicating that no corresponding physical storage space has yet beenallocated, or results in a reference to a data device which in turnreferences the underlying physical storage space.

Referring to FIG. 4A, shown is a diagram illustrating an exemplarylogical division of a storage of a data storage system into storageobjects (such as RAID groups) for managing data relocation in the datastorage system that may be included in an embodiment using thetechniques described herein. With reference also to FIGS. 1 and 2, forexample, storage entities 102 may refer to either a single storagedevice or a RAID group operating as a single storage device, may befurther sub-divided into logical units. A single RAID group orindividual storage device may contain one or more logical units (LUs)42. However, RAID groups need not correspond to LUs and RAID groupingsmay be further divided into two or more LUs. In addition to RAID groups,each logical unit 42 may be further subdivided into portions of alogical unit, referred to as “slices” 114. Slices 114 may be allocated,de-allocated, re-allocated, reserved, or redistributed by a slicemanger. A slice may be, for example, a 1 GB slice of data. Further, aslice may be, for example, a 256 MB slice of data. However, thetechniques described herein should not be construed as being limited toonly slices of data; the techniques are equally applicable to other datachunk sizes, such as blocks, slivers (subset of slices), page, file orthe like. The slice manager may be a software application or layer thatis executed, at least in part, by one or more SPs 46A, 46B. The slicemanager may be responsible for implementing a slice allocation policyand/or algorithm. For example, the slice manager may receive sliceallocation requests, and maintain relevant statistical informationregarding slices.

Referring to FIG. 4B, shown is a diagram illustrating another example oflogical division of a storage of a data storage system into storageobjects (such as RAID groups, storage devices, slices) for managing datarelocation in the data storage system that may be included in anembodiment using the techniques described herein. In at least oneembodiment, a collection of hard disk drives may be organized into RAIDarrays. The collective data storage capacity of storage devices (e.g.,RG1 110A) is represented by data storage space. The data storage spacemay be divided into portions, hereinafter referred to as slices 114(e.g., SLICE1-SLICE10). In at least one embodiment of the currenttechnique, for example, each slice 114 is approximately 1 gigabyte (GB)in size, but other sizes may be used. Slices 114 within the data storagespace may be organized into logical units (LUs), which are commonlyreferred to as LUNs.

In at least one embodiment of the current technique, data storagesystems that comprise storage devices of varied performancecharacteristics grouped into tiers can be managed in such a way as tomigrate data from one portion of the storage pool to another portion ofthe storage pool. A particular embodiment may help achieve thismigration by automatically migrating data among the tiers based on the“temperature” of contents of a slice and location of the slice onstorage devices. In general, temperature may correspond to, for example,how often and how recently the data is accessed. For example, hot datamay refer to data that has been accessed recently and is accessed often,cold data may refer to data that has not been accessed recently and isnot accessed often. Data temperature may be further segmented to includea warm data category that may include data that is less hot than hotdata and/or less cold than cold data. Hence, warm data may refer to datathat is accessed more often than cold data and less often that hot data.In general, hot data is migrated to faster (and typically moreexpensive) storage, and cold data is migrated to slower (and typicallyless expensive) storage. Warm data may be migrated to either type ofstorage and such storage may be configurable to be placed in a reducedpower consumption state. Generally, migration maybe accomplished bycopying the data and changing the map entries for the logical addressedthat were involved to reflect the new logical to physical association.Thus, hot data may be stored in disk drives indicated as hot disks andcold data may be stored in disk drives indicated as cold disks.

Additional details regarding slice relocation and tiered data storagearrays are disclosed in U.S. patent application Ser. No. 12/826,434,filed on Jun. 29, 2010 and entitled, “MANAGING MULTI-TIERED STORAGE POOLPROVISIONING” and U.S. patent application Ser. No. 12/824,816, filed onJun. 28, 2010 and entitled, “METHODS, SYSTEMS, AND COMPUTER READABLEMEDIUM FOR TIER-BASED DATA STORAGE RESOURCE ALLOCATION AND DATARELOCATION IN A DATA STORAGE ARRAY” which are incorporated by referenceherein in their entireties.

Referring now to FIG. 5 that illustrates a process of relocating slicesfrom a first storage tier of a storage pool to a second storage tier ofthe storage pool in a data storage system. In this figure, there arethree storage tiers, Tier 1, Tier 2 and Tier 3. Each storage tierincludes slices, such as slices 300, 310, and 320. As well, each slicehas a temperature associated with it such as hot, cold, or medium. Aswell, some of the storage tier is also considered empty. Referring tothe upper portion of the FIG. 5, there is a hot slice 310 in storageTier 2. The temperature of a slice may be designated as a scalar or stepvalue that is it may have a numerical equivalent such as 30 degrees ormay simply be designated into a bucket, such as cold.

Also shown in the FIG. 5 is that Tier 1 has empty space 300. In thisexample, Tier 1 may have faster performance characteristics and a highercost. Conversely, Tier 2 may have slower performance characteristics buta lower cost. This may be seen, for example, in the fact that there ismore storage in Tier 2 than there is in Tier 1. Again, in the upperportion of the FIG. 5, it is shown that there is a hot slice 310 in Tier2 that should be moved to Tier 1. In this example embodiment, as shownin the lower portion of FIG. 5, the hot slice is moved to Tier 1 leavingan empty space 340 in Tier 2.

Referring now to FIG. 6 that illustrates a process of relocating slicesfrom a first storage tier of a storage pool to a second storage tier ofthe storage pool in a data storage system. In this embodiment, there isa hot slice 410 in Tier 2 and a medium slice 400 in Tier 1; however,Tier 1 has no space to accommodate an additional tier. Therefore, inthis embodiment, the medium slice 400 on Tier 1 is migrated to Tier 2and the hot slice 410 in Tier 2 is migrated to Tier 1. Note, that it wasthe need to migrate the hot slice 410 to Tier 1 that caused the mediumslice 400 to be shifted to Tier 2. In this example, it may have beenmore effective to have the medium slice located in Tier 1. Also notethat slices may change temperature based on data access requests.Therefore, a slice's temperature may rise or fall over time. The slice'stemperature may be the result of any number of calculations based ondata access or data write requests to that slice.

Referring to FIG. 7, shown is more detailed example of an embodiment ofa computer system that may be used in connection with performing thetechniques described herein. With reference also to FIGS. 1-3, in a datastorage system such as data storage system 12, a storage processorprovides communications between host 14 and disk drives 60. Data storagesystem 12 includes at least two storage processors 46A, 46B. Bothstorage processor A (SPA) 46A and storage processor B (SPB) 46A providesaccess to Flare LUNs 105-108 built from a storage space provided by diskdrives 60. Generally, a Flare LUN can only be accessed by one storageprocessor. Lower redirector 102 interacts with storage processors 46A,46B to access Flare LUNs 105-108. The access to Flare LUNs 105-108 isindependent of which storage processor each Flare LUN belongs to. A userof data storage system 12 allocates storage from Flare LUNs in fixedsized chunks. Each fixed size chunk is known as a slice. One or moreslices are grouped together to create a slice pool. Host system 14provisions storage from slice pools 100 for creating mapped LUNs 81-84.A mapped LUN is a LUN that is visible to host system 14 and a user of adata storage system. A mapped LUN may be a thin LUN (TLU) or a directLUN (DLU). The size of a thin LUN is independent of amount of availablestorage. Typically, storage is allocated to a thin LUN when host system14 issues a write request and needs a data block to write user's data.The size of a direct LUN is dependent of amount of available storage.Typically, storage is allocated to a direct LUN at the time the directLUN is created and initialized. File system mapping driver 85 is alight-weight file system library that provides file system functionalityand allows data storage system 12 to create files within a file system.File system mapping driver 85 processes I/Os directed to metadata of afile system. Mapped LUN driver 80 processes I/Os directed to data of thefile system. Mapped LUN driver 80 also provides slices of storage fromslice pools 100 to file system mapping driver 85 for creating a filesystem. Slices of storage can be dynamically added or removed by a filesystem. When a slice is removed, the file system redistributes datastored on the slice to other slices in the file system. File systemmapping driver 85 allocates file system blocks from slices of storagefor creating files and storing metadata of a file system. In at leastsome embodiments of the current technique, size of the file system blockmay be 8 kilobyte (KB) in size. A sparse volume concatenates slices ofstorage provided to file system mapping driver 85 into a logicalcontiguous address space on which a file system is created. The sparsevolume maintains logical to physical mapping for slices of storage usedto create the file system. Further, the file system maintains anallocation bitmap for every slice of physical storage that is used tocreate the file system. A mapped LUN presents a file as a LUN to hostsystem 11. Further, the file presents a contiguous logical address spaceto the mapped LUN. For example, in FIG. 7, mapped LUN 81 presents file86 as a LUN to host system 11, file 86 is created in a file system 90and file system 90 is created from sparse volume 95. Similarly, mappedLUNs 82-84 presents file 87-89 as LUNs respectively to host system 11,files 87-89 are created in file systems 91-93 respectively and filesystems 91-93 are created from sparse volumes 96-98 respectively.Further, sparse volumes 95-98 are created from slices of physicalstorage included in slice pools 100.

Referring to FIG. 8, shown is more detailed representation of a filesystem mapping driver 85 that may be included in an embodiment using thetechniques herein. Sparse volume 95 aggregates one or more slices ofphysical storage together into a contiguous logical address space whilesome of these slices may or may not be provisioned. A provisioned slicehas physical storage space allocated for storing data in the provisionedslice. For example, in FIG. 8, sparse volume 95 aggregates slices125-134 together into a logical address space of 16 gigabyte (GB), whereeach slice is 1 gigabyte (GB) in size. However, it should be noted thateach slice may be 256 megabyte (MB) in size. Root slice 125 and Slice-0126 in a sparse volume is always provisioned, such that a storage spaceis available to store metadata information for slices included in thesparse volume 95. File system 90 is created from the contiguous logicaladdress space provided by the sparse volume 95. A user of data storagesystem 12 creates files 86, 116-118 in file system 90. Each provisionedslice of a sparse volume has a corresponding configured slice objectthat is mapped to a corresponding LUN of physical storage included indevice volumes 60. In at least some implementations, root slice 125 isstored in a data portion of slice-0 126, but for generality, the rootslice is defined independently of the slice-0. Additionally, root slice125 holds logical to physical address mapping for sparse volume 95.

At any given time, a storage space for a file system is either allocated(also referred to as provisioned) or not. If a storage space for a filesystem is not allocated, then there is said to be a hole at thatlocation in a logical extent of the file system. For example, in FIG. 8,logical address space of sparse volume 95 has four holes 127, 130, 132,133 indicating that slices corresponding to those locations are notprovisioned.

Referring to FIG. 9, shown is a more detailed representation ofcomponents that may be included in an embodiment using the techniquesdescribed herein. With reference also to FIG. 8, root slice 125 of asparse volume 95 includes metadata of the sparse volume 95. Root slice125 includes metadata information such as root slice head 140 pointingto the first slice of a set of slices included in the sparse volume 95,and slice map sector 142 which includes metadata entries such as a slicemap entry 160 for each slice of the set of slices included in the sparsevolume 95. For example, as shown in FIG. 9, slice map sector 142includes slice map entries for slices 143-148 (other slices not shown)included in the sparse volume 95. A slice map entry such as slice mapentry-0 143 for a slice includes metadata information for the slice. Themetadata information included in the slice map entry 143 for a sliceincludes device identification number 161 indicating a physical deviceon which the slice resides, offset 162 indicating the offset at whichthe slice resides on the physical device, and file system identificationnumber 163 indicating the file system to which the slice belongs.Further, in at least one embodiment of the current technique, newmetadata, empty slice bit 41, is added to slice map entry 143 whichhelps determine whether the slice is an empty slice indicating that theslice does not include valid user data.

Referring to FIG. 10, shown is a more detailed representation ofcomponents that may be included in an embodiment using the techniquesdescribed herein. In at least some embodiments of the current technique,a storage pool may include one or more RAID groups. A RAID group may beassociated with data devices, such as the physical devices 60 a-60 bdiscussed herein, so that, for example, there is one or more datadevices for each RAID group, any portion of a data device for anyportion of the pools of storage, and/or any combinations thereof.Further, data devices associated with a storage pool may have differentcharacteristics, such as speed, cost, reliability, availability,security and/or other characteristics. Further, storage pool 102 mayinclude one or more storage tiers 458, 460, 462 such that each storagetier has different performance characteristics.

In at least one embodiment of the current technique, slice relocationmanager 452 (also referred to as “Auto-Tiering policy engine (PE)”) mayshift hot slices of a logical volume to upper tiers and cold slices ofthe logical volume down to lower tiers. The goal of the slice relocationprocess is to put hot, frequently accessed slices to higher tiers andmaximize the utilization of these high tiers, which include faster butmore expensive drives such as a flash storage drives. Slice relocationmanager 452 relocates a slice based on the temperature of the slice. Thetemperature of a slice is determined based on I/O activity directed tothe slice. I/O activity of a slice is an indicator of current I/O loadof the slice. Slice I/O activity is computed using raw slice statistics.The computation may be done in any of several different ways. Thespecific computation to use may depend on the system I/O trafficpattern. In at least some cases, the simplest and most straightforwardcalculation is to use total slice I/O counts as I/O activity, such thatthe slice I/O load is the moving average of slice I/O counts.

In at least one embodiment of the current technique, slice relocationmanager 452 works in conjunction with file system mapping driver 85 torelocate slices. Further, file system mapping driver 85 works inconjunction with sparse volume management logic 454 to relocate slicesselected for relocation by slice relocation manager 452. Sparse volumemanagement logic 454 evaluates a slice using metadata information suchas an empty slice bit included in a slice map entry for the slice anddetermines whether the slice is provisioned, unused and unallocated orprovisioned and in use. The empty slice bit included in a slice mapentry of a slice indicates whether the slice has been written to suchthat sparse volume management logic 454 may skip copying data for theslice and relocate the slice to a destination slice without having tocopy contents of the slice. Thus, in at least one embodiment of thecurrent technique, copying contents of an empty slice which does notinclude any valid user data is skipped when relocating the empty slice.Sparse volume management logic 454 maintains metadata information suchas a bitmap indicating whether a slice is empty. Sparse volumemanagement logic 454 evaluates the bitmap when relocating slices.Further, when data source 450 sends an I/O directed to a slice, sparsevolume management logic 454 updates the bitmap (e.g., empty slice bit164) for the slice indicating that the slice has been written to, is notempty and includes valid user data. Thus, in at least one embodiment ofthe current technique, slice relocation manager 452 working inconjunction with file system mapping driver 85 sends a list of slices tosparse volume management logic 454 for relocation. Sparse volumemanagement logic 454 evaluates each slice of the list of slices anddetermines whether each slice is empty and only performs copying ofcontents of a slice during relocation upon determining that the slice isnot empty. Further, in at least one embodiment of the current technique,a user may configure whether to skip copying data of an empty slice.Further, bitmap indicating whether a slice is empty may be maintainedboth in a memory of a storage system and on a disk such that sparsevolume management logic 454 may determine whether a slice is empty forminformation stored in the memory and flush the information maintained inthe memory to the disk at a later time.

Further, in at least one embodiment of the current technique, when aslice is initialized, the empty slice bit for the slice is set to zeroin the slice map entry associated with the slice. When a first write I/Orequest is received for the slice, the empty slice bit for the slice isset to 1 in the slice map entry for the slice. Thus, during relocationof a slice, copying of data of the slice is skipped if the empty slicebit for the slice is set to 0 and in such a case, a destination slice issimply marked as the newly relocated slice without having to copycontents of the slice to the destination slice. Further, when a slice isrecovered in case of a failure, the empty slice bit for the slice may bereset.

Referring to FIG. 11, shown is a more detailed flow diagram illustratingmanaging data relocation in storage systems. With reference also to FIG.10, a data storage system manages data relocation of a slice (step 470).Metadata of a slice selected for relocation is evaluated (step 472).Based on the metadata, a determination is made as to whether the sliceis empty or includes user data (step 474). Upon determining that theslice is empty, copying data of the slice is skipped during relocationof the slice (step 476). However, upon determining that the slice is notempty but includes user data, the slice is relocated by copying contentsof the slice to a destination slice (step 478).

While the invention has been disclosed in connection with preferredembodiments shown and described in detail, their modifications andimprovements thereon will become readily apparent to those skilled inthe art. Accordingly, the spirit and scope of the present inventionshould be limited only by the following claims.

What is claimed is:
 1. A method for use in managing data relocation instorage systems, the method comprising: evaluating metadata of eachslice of a set of slices of a storage tier in a data storage system formigrating the set of slices from the storage tier to another storagetier, wherein the metadata of each slice of the set of slices isincluded in a slice map, wherein the slice map includes a set of slicemap entries for the set of slices, each slice map entry of the set ofslice map entries including the metadata for a respective slice of theset of slices and an empty slice indicator for the respective slice ofthe set of slices, wherein the empty slice indicator for a sliceindicates whether the slice includes user data, wherein the empty sliceindicator for a slice is updated to indicate the slice as an empty sliceupon provisioning of the slice to a logical volume, wherein the emptyslice indicator for an empty slice is subsequently updated uponreceiving a first request to write user data to the empty slice, whereinan empty slice indicates a slice that is provisioned to a logical volumeand does not include user data, wherein the data storage system includesa first storage tier and a second storage tier configured such thatperformance characteristics associated with the first storage tier issuperior to the second storage tier; and based on the evaluation,avoiding copying contents of a slice of the set of slices to adestination slice upon determining that the slice is an empty slice,wherein metadata of the destination slice is updated to indicate thatthe slice has been migrated to the destination slice without having tocopy contents of the slice.
 2. The method of claim 1, wherein anauto-tiering policy engine identifies a set of slices for relocation,wherein the slice is a logical representation of a subset of physicaldisk storage.
 3. The method of claim 1, wherein the storage tier and theanother storage tier includes a disk drive system comprising a pluralityof Redundant Array of Inexpensive Disks (RAID) systems, each RAID systemof the plurality of RAID systems having a first disk drive and a seconddisk drive.
 4. The method of claim 1, wherein the empty slice indicatorincludes an empty slice bit.
 5. The method claim 4, further comprising:updating the empty slice bit associated with the slice upon receiving afirst write I/O request for the slice, wherein the empty slice bitindicates that the slice includes user data.
 6. The method claim 4,further comprising: updating the empty slice bit associated with theslice upon initialization of the slice, wherein the empty slice bitindicates that the slice does not include user data.
 7. The method ofclaim 1, further comprising: determining whether the slice includes userdata; and based on the determination, relocating the slice withoutcopying contents of the slice to a destination slice.
 8. A system foruse in managing data relocation in storage systems, the systemcomprising: first logic evaluating metadata of each slice of a set ofslices of a storage tier in a data storage system for migrating the setof slices from the storage tier to another storage tier, wherein themetadata of each slice of the set of slices is included in a slice map,wherein the slice map includes a set of slice map entries for the set ofslices, each slice map entry of the set of slice map entries includingthe metadata for a respective slice of the set of slices and an emptyslice indicator for the respective slice of the set of slices, whereinthe empty slice indicator for a slice indicates whether the sliceincludes user data, wherein the empty slice indicator for a slice isupdated to indicate the slice as an empty slice upon provisioning of theslice to a logical volume, wherein the empty slice indicator for anempty slice is subsequently updated upon receiving a first request towrite user data to the empty slice, wherein an empty slice indicates aslice that is provisioned to a logical volume and does not include userdata, wherein the data storage system includes a first storage tier anda second storage tier configured such that performance characteristicsassociated with the first storage tier is superior to the second storagetier; and second logic avoiding copying, based on the evaluation,contents of a slice of the set of slices to a destination slice upondetermining that the slice is an empty slice, wherein metadata of thedestination slice is updated to indicate that the slice has beenmigrated to the destination slice without having to copy contents of theslice.
 9. The system of claim 8, wherein an auto-tiering policy engineidentifies a set of slices for relocation, wherein the slice is alogical representation of a subset of physical disk storage.
 10. Thesystem of claim 8, wherein the storage tier and the another storage tierincludes a disk drive system comprising a plurality of Redundant Arrayof Inexpensive Disks (RAID) systems, each RAID system of the pluralityof RAID systems having a first disk drive and a second disk drive. 11.The system of claim 8, wherein the empty slice indicator includes anempty slice bit.
 12. The system claim 11, further comprising: thirdlogic updating the empty slice bit associated with the slice uponreceiving a first write I/O request for the slice, wherein the emptyslice bit indicates that the slice includes user data.
 13. The systemclaim 11, further comprising: third logic updating the empty slice bitassociated with the slice upon initialization of the slice, wherein theempty slice bit indicates that the slice does not include user data. 14.The system of claim 8, further comprising: third logic determiningwhether the slice includes user data; and fourth logic relocating, basedon the determination, the slice without copying contents of the slice toa destination slice.